Sub Panel extra separate earth ground?

I have a building 100 feet from my house. The electrician is burying a cable to install a sub panel in the building. He is feeding it from the main panel in my house. He will be running 3 conductors and a ground from the main panel. The sub panel will have a ground wire leading back to the main panel ground. He wants to install an earth ground rod at the sub panel just outside the building. Is it right to install this extra earth ground or should the main box have the only earth ground?

Required in 250.32(A). The building is required to connected to a grounding electrode system, in addition to the equipment grounding conductor required in 250.32(B). The grounding electrode conductor has to be connected to the equipment grounding conductor at the building.

A very smart man once explained it to me like this. "Consider the house as an island. And consider the second building as another island. You need to re-establish a grounding electrode system at the second building."

You must have a grounding electrode [system] at the second building, connected to the equipment ground terminal bar of the subpanel. The neutral must float and not be bonded to the subpanel frame. If the new grounding electrode system consists solely of one ground rod, then it does not meet code. You would need at least two ground rods spaced at least 6-feet apart (preferably a bit more).

(2) Supplemental Electrode Required. A single rod, pipe, orplate electrode shall be supplemented by an additional elec‐trode of a type specified in 250.52(A)(2) through (A)(8). Thesupplemental electrode shall be permitted to be bonded to oneof the following: